1. Field of the Invention
The present invention relates to a sewing machine having a function of tracing the edge of a workpiece, and more particularly to such an edge tracing sewing machine wherein the detection area of an edge sensor is offset a certain distance away from a lowered position of a sewing needle (hereinafter referred to as "lowered needle position", if appropriate) in the feed direction of the workpiece.
2. Discussion of the Prior Art
An edge tracing sewing machine is known according to laid-open Publication No. 61-257675,the subject matter of which is incorporated in U.S. Pat. No. 4,664,048. In this sewing machine, the edge of a workpiece or work fabric is detected by an arrangement which includes (i) a feeding device for feeding the workpiece in a predetermined feed direction, in synchronization with an endwise reciprocable sewing needle, (ii) a control motor for changing a relative position between the needle and the workpiece in a lateral direction perpendicular to the feed direction of the workpiece, (iii) an edge sensor having a detection area on a work bed, and generating an output signal corresponding to a position relative to an edge of the workpiece in the detection area in the lateral direction, and (iv) position control means for receiving the output signal of the edge sensor in synchronization with the endwise reciprocations of the needle, and applying to the control motor a drive signal to control the relative position between the needle and the workpiece endge. The drive signal is determined based on the output signal of the edge sensor, so that successive stitches are formed along the workpiece edge.
In the edge tracing sewing machine of the type described above, it is physically difficult to dispose the edge sensor such that the position at which the edge sensor detects the workpiece edge is aligned with a stitch forming position of the machine in the direction of feed of the workpiece. For this reason, the detection area of the edge sensor is usually offset a certain distance away from a lowered position of the sewing needle (i.e., stitch forming position) on the upstream side in the feed direction of the workpiece.
Accordingly, the portion of the workpiece which is detected by the edge sensor (detected portion of the workpiece) is different from the portion of the workpiece which is sewn by the needle whose lateral position is regulated according to an output signal of the edge sensor. In the case where stitches are formed along a straight line parallel to the workpiece feed direction, the fact indicated above does not cause a problem. However, in the case of a sewing operation wherein stitches are formed along a curved edge of the workpiece, the distance between the stitch positions and the workpiece edge tends to be fluctuated if the edge sensor or detected portion of the workpiece is offset from the lowered needle position or stitch forming position of the workpiece.